1. Field of the Invention
The present invention relates to technology for controlling the heat value of heating elements in a thermal head.
2. Description of the Related Art
Generally speaking a thermal head produces heat by independently passing a specified current to each heating element in order to form a dot pattern on thermal paper. The amount of heat is controlled by regulating the energizing time, or more specifically the energizing pulse width.
The thermal head also has a heat storage characteristic such that heat accumulates as the current supplied to a same heating element is maintained. This brings about the use of a characteristic called energizing history, which is a determination of the heat stored in thermal head based on the amount of energizing time, i.e. current, previously supplied to the thermal head. Thermal history control is therefore applied to keep the heat output from the heating element constant by shortening the energizing pulse width according to its energizing history.
This thermal history control functions by transferring one line of print data for each heating element row to a print buffer for temporary storage and to a history buffer for storage as history data. A logic operation is then performed on the print buffer data and history buffer data in order to determine the energizing pulse width for the next print data for each heating element; a short pulse width is sent to the thermal head for application to heating elements that operated (i.e. were energized for printing) immediately before, and a normal pulse width is sent to the thermal head for application to heating elements that did not print (i.e. were not energized) immediately before.
Thermal paper has a paper base with a heat-sensitive coating that produces color when a certain amount of heat is applied. Two-color thermal paper that produces different colors according to the difference in the applied heat is also known.
When this two-color thermal paper is used with the above-described thermal head, it is desirable to be able to selectively use a two-color mode for printing two colors (such as black and red) and a monochrome mode for printing only one color (such as black).
To achieve this, it is necessary to selectively generate both long energizing pulse widths for high heat output and short energizing pulse widths for low heat output when using the two-color mode.
The problem with this is that the circuit for controlling the energizing of the thermal head""s heating elements becomes complex when using two-color thermal paper.
A further problem with two-color printing is that the low-heat output color appears like a ghost around the edges of the high-heat output color.
Therefore, it is an object of the present invention to overcome the aforementioned problems.
It is a further object of the present invention to provide a two-color printing control circuit/method that eliminates, or significantly reduces, ghost effect of the low-heat output color around the high-heat output color.
The present invention is directed to solving these technical problems and an object of the invention is to provide a control apparatus and a control method for a thermal head capable of printing two colors and applying thermal history control while featuring a simple circuit configuration.
One aspect of the present invention assumes that the above described problems are at least partly caused by insufficient time for distributing heat evenly across the heating elements and on to the thermal paper.
To achieve these objects, a thermal head control method for controlling different heat output levels by changing amount of energizing applied to heating elements of the thermal head, comprising the steps of: (a) energizing heating elements in a first energizing stage and in a second energizing stage to produce a first color at a first heat output level, and (b) providing a pause of a predetermined duration between the first energizing stage and the second energizing stage.
When printing with the color produced by the first heat output level to the same dot on the thermal paper, the present invention separately applies the first and second energizing stages to assure the heat output required to produce the color emitted at the first heat level, which actually prints the same dot twice. A sharp, clear first color print image can thus be achieved.
Further preferably, the pause of the predetermined time period is of sufficient time for the second energizing stage to distribute the first heat output level substantially throughout the entire surface of the heating elements.
Thus the temperature of the heating elements distributed to thermal paper and becomes substantially even on it. A sharp, clear print image can be achieved.
Yet further preferably, the first energizing stage is greater than the second energizing stage.
By thus setting the first stage heat level higher than the second stage heat level, the same dot on the thermal paper is first printed using a dark color and is printed a second time using a lighter color, thereby assuring a well-defined print image. It is also possible to rapidly heat the thermal head heating elements when printing to thermal paper using the first heat output level.
The invention also provides a thermal head control method for energizing heating elements only in the second energizing stage to produce a second color at a second heat output level.
When printing with the color produced by the second heat output level, the present invention can thus sufficiently cool the heating elements of the thermal head by energizing the thermal head only in the second stage and not in the first stage. It is therefore not necessary to consider thermal history.
Further preferably, the energizing level in the second energizing stage is sufficient to produce a second color at a second heat output level.
In the second stage, the heat output is required to produce the color emitted at the second heat level. A sharp, clear second color print image can thus be achieved.
To further achieve the above objects, the present invention also provides a thermal head control apparatus according to the present invention for controlling heat output from heating elements of the thermal head by changing the energizing of each heating element in the thermal head, which has an array of multiple independently drivable heating elements. The thermal head control apparatus has a print data processing unit for processing a print data group that is information relating to producing color at a specific heating element according to different heat output levels, and a drive control circuit. The print data group contains both or either first color data based on a first heat output level and second color data based on a second heat output level that is lower than the first heat output level. The print data processing unit has a first command processing function for converting first color data to a first stage energizing command and a second stage energizing command, a second command processing function for converting second color data to a second stage energizing command, a first memory area for storing the first stage energizing command, and a second memory area for recording the second stage energizing command, and selectively outputs energizing commands from the first memory area and energizing commands from the second memory area. The drive control circuit has a first command storage area for storing energizing commands contained in the first memory area or second memory area, and a second command storage area for storing energizing commands contained in the first command storage area, and energizes a specific heating element by means of a energizing command based on a comparison of content stored to the first command storage area and the second command storage area.
The invention thus comprised sets aside first and second memory areas in the print data processing unit, and applies the result of an operation on the first color data and second color data to the drive control circuit. The output temperature of the thermal head heating elements can therefore be held constant with consideration for thermal history even when the drive control circuit is a circuit enabling thermal history control for monochrome printing, and the heat output required to produce the first color and the heat output required to produce the second color can be separately controlled and achieved in the individual heating elements of the thermal head.
It is therefore possible to provide a control apparatus featuring a simple circuit configuration and applying thermal history control for a thermal head capable of printing two colors.
Preferably, the first command processing function of the print data processing unit extracts first color data from the print data group to a first working area and stores it to the first memory area, and the second command processing function extracts second color data from the print data group to a second working area and stores the result of a logic operation on the first working area and second working area to the second memory area.
First stage information can therefore be recorded in a first memory area for first color output data, and second stage information can be stored in a second memory area for first and second color output data. As a result, information relating to the heat output required to produce the first color can therefore be applied to the drive control circuit in first and second stages, and information relating to the heat output required to produce the second color can be applied to the drive control circuit as second stage data.
Further preferably, the print data processing unit converts first stage energizing commands and second stage energizing commands to multiple energizing commands stored to the memory areas based on print data and the history of energizing the heating elements.
Thus comprised, heat output for producing the first color can be controlled using a first stage energizing command and a second stage energizing command, and heat output for producing the second color can be controlled using the second stage energizing command.
It is yet further effective for the multiple first and second stage energizing commands to be energizing pulses, and determine energizing pulse width according to the heat output required to produce the first color.
Thus comprised the heat output required to produce the first color can be controlled using the energizing pulse width determined by a plurality of first and second stage energizing commands.
It is also effective to determine the energizing pulse width of the multiple second stage energizing commands according to the heat output required to produce the second color.
Thus comprised the heat output required to produce the second color can be controlled using the energizing pulse width determined by a plurality of second stage energizing commands.
Yet further preferably, the initial first stage energizing command is determined according to the relationship between heat output due to multiple second stage energizing commands and the cooling temperature of the thermal head heating element.
Thus comprised the energizing pulse width of the initial first stage energizing command can be appropriately changed, and the pause of the predetermined time period, so that the temperature generated in the first stage and second stage is flat for a particular heating element of the thermal head. A sharp, clear first color print image can thus be achieved.
Yet further preferably the initial second stage energizing command is determined according to the relationship between heat output due to multiple first stage energizing commands and the cooling temperature of the thermal head heating element.
Thus comprised the energizing pulse width set by the initial second stage energizing command can also be appropriately changed so that the temperature generated in the second stage is flat for a particular heating element of the thermal head. A sharp, clear second color print image can thus be achieved.
Yet further preferably, the drive control circuit runs a last first stage energizing command or last second stage energizing command directly from the command storage area based on the print data, and runs energizing commands other than the last first stage energizing command or last second stage energizing command based on a NOT-AND operation between a command storage area written according to the energizing print data and a previous-print-data command storage area.
Thus comprised, it is also possible to selectively energize the heating elements of the thermal head in order to achieve the basic heat output level required to produce the first and second colors, and to achieve the heat output required to produce the first and second colors with consideration for thermal history.
Yet further preferably, the drive control circuit comprises a energizing output circuit for outputting an N-th strobe signal at a specific timing for an N-th energizing command (where N is a positive integer).
Thus comprised the multiple first and second stage energizing commands can be easily generated.
Yet further preferably, the control apparatus uses the drive control circuit for monochrome printing, and by using only the first stage energizing command prints one line using half the energizing commands used for two-color printing.
Thus comprised the invention also achieves a thermal head printer using a monochrome printing drive circuit applying thermal history control to enable sharp, clear monochrome and two-color printing.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.